Understanding the rate of lymphocyte replication and destruction in HIV-infected patients, as well as the effects of therapy on lymphocyte replication should lead to a better understanding of the mechanisms behind the immunodeficiency induced by HIV. Little is known about the replication rate in healthy and HIV-infected patients. Two approaches are being used to address this issue. (1) Healthy and HIV-infected patients will receive up to 5 days of continuous infusions with [6,6-2H2]-glucose, a nonradioactive, stable isotope of glucose that is safe to administer. The deuterium is incorporated into DNA via metabolism of glucose to ribose and incorporation into nucleotides. The rate of incorporation can be measured in subpopulations of cells to determine the rate of replication of those cells, and the rate of loss of the incorporated deuterium can be used to examine the turnover rate of the replicated cells. (2) Administering bromodeoxyuridine (BrDU; 200 mg/m2), an analogue of thymidine, to HIV-infected patients. BrDU is incorporated into DNA and incorporation can be measured using an anti-BrDU monoclonal antibody. By FACS analysis, both surface markers and BrDU can be measured. Thus, FACS analysis can be used to directly measure subpopulations of cells that have replicated. To date, 35 patients have been enrolled in these studies. Techniques for measuring incorporation have been developed and validated for both methods. Studies with BrDU have identified two populationsof proliferating cells, one with a rapid turnover, and the second with a slow turnover. The size of the rapidly proliferating pool, but not the slowly proliferating pool, is directly related to the log viral load, suggesting that HIV drives cells to enter the rapidly proliferating pool. Studies are ongoing to follow up on these observations and to evaluate lymphocyte replication in other settings. These two approaches should provide information about lymphocyte kinetics that will have relevance to HIV infection and other disease states.